State-of-the-art photovoltaic devices are based on a PN junction in a semiconductor across which the photovoltage develops. Notwithstanding the non-ideal characteristics, silicon remains at present the semiconductor most used for photovoltaic conversion.
Solid-state physics shows that for a 90% light absorption, just 1 μm of GaAs (a direct semiconductor) would be necessary compared to 100 μm of monocrystalline Si. However, because of the well-developed techniques in fabricating silicon devices, silicon remains the most used semiconductor material, even for photovoltaic conversion.
The constant drive to increase higher and higher energy conversion efficiencies have led to several technological approaches for fabricating efficient photovoltaic cells. One of these approaches that is generally considered particularly promising is based on transferring thin monocrystalline silicon films from the surface of a “mother” silicon wafer onto a dielectric substrate. The basic idea of this approach is to detach a partially processed thin monocrystalline silicon layer from the surface of a common FZ-wafer and to transfer it onto a glass substrate.
The possibilities offered by fabrication techniques developed for integrating on silicon certain passive electrical and mechanical elements through a MEMS (Micro Etching Machining of Silicon) technology potentially permit definition of highly efficient integrated cell geometries of high efficiency. Moreover, the same starting wafer may be used several times as the “mother” wafer for forming a new light-trapping layer after polishing the release surface of the mother wafer and re-growing thereon a new epitaxial layer.
The known techniques of fabrication of photovoltaic panels proposed so far have significant drawbacks and shortcomings because of the processing complexity and cost to form a layer of silicon that may be safely detached from the surface of a monocrystalline mother wafer and transferred on a substantially rigid transparent dielectric substrate. The substantially rigid transparent dielectric substrate typically comprises a glass plate to which the detachable thin layer of silicon becomes permanently associated. This produces significant limitations on the ensuing steps of the fabrication process of photovoltaic panels because of the rigidity and unreplaceability of the transparent dielectric substrate on which the intrinsically fragile, partly defined thin crystalline silicon layer structure permanently bonded thereto is subjected to during the final steps of fabrication.
Considering optimal standard conditions, the radiation power at sea level per unit of area is about 1 kW/m2, and considering a 20% efficiency for a monocrystalline silicon based photovoltaic panel, for a power yield of 1 kW, at least 5 m2 of monocrystalline silicon are needed, which equals about 225 6-inch wafers, accounting for the overall area of typically round substrates. This implies a cost of USD 4000-5000 for just raw material of good quality. It is therefore evident there is a need of devising new fabrication processes capable of drastically reducing the overall costs of photovoltaic cells of enhanced efficiency realized on a monocrystalline silicon substrate.